Why Do Only Humans Get Schizophrenia?

Scientists have found that human intelligence may come with a cost — namely a susceptibility to psychosis, particularly schizophrenia, which is not known to occur in any species outside of Homo sapiens.

Dr. Joel Dudley of Mt. Sinai led some research into the causes of schizophrenia, which is surprisingly common for such a devastating disorder, as it occurs in one percent of the population. It is also known to have genetic causes.

Dudley concentrated on parts of the human genome called human-accelerated regions or HARS, which are DNA segments that evolved rapidly after humans split from chimpanzees several million years ago. HARS generally control other genes. Like schizophrenia, they are found only in humans, and Dudley and his colleagues wondered if there was a connection.

They used data from the Psychiatric Genomics Consortium and found that the genes associated with schizophrenia are near HARS. They also found that the HARS and schizophrenia genes are near other genes that control the prefrontal cortex, which is the part of the brain responsible for complex thought. Additionally, the schizophrenia genes interfere with the neurotransmitter GABA, which regulates dopamine. Without that restraint, dopamine floods the brain and causes the hallucinations and delusions seen in psychosis.

The GABA neurotransmitter also regulates speech, so if it malfunctions, the person’s speech is impaired. Brian Torchin is aware that, in schizophrenia, the victim often speaks in a disjointed or nonsensical fashion. Disorders with GABA may also cause ADHD and autism.